The present invention relates generally to feature measurement in scanning electron microscopy, and more specifically to apparatus and methods for enhancing image quality. The present invention may also be applied to feature measurement and image enhancement in similar instruments.
FIG. 1 is a diagrammatic representation of a conventional scanning electron microscopy configuration 100. As shown, a beam of electrons 102 is scanned over a specimen 104 (e.g., a semiconductor wafer). Multiple raster scans 112 are typically performed over a small area 114 of the specimen 104. The beam of electrons 102 either interact with the specimen and cause an emission of secondary electrons 106 or bounce off the specimen as backscattered electrons 106. The secondary electrons and/or backscattered electrons 106 are then detected by a detector 108 that is coupled with a computer system 110. The computer system 110 generates an image that is stored and/or displayed on the computer system 110.
Although conventional microscopy systems and techniques typically produce images having an adequate level of quality under some conditions, they produce poor quality images of the specimen for some applications. For example, on a specimen made of a substantially insulative material (e.g., a semiconductor material), performing multiple scans over a small area sometimes causes the specimen to accumulate excess positive charge in the small area relative to the rest of the specimen. The excess charge generates a potential barrier for some of the secondary electrons, and this potential barrier inhibits some of the secondary electrons from reaching the detector 108. Since this excess charge is likely to cause a significantly smaller amount of secondary electrons to reach the detector, an image of the small area is likely to appear dark, thus obscuring image features within that small area.
Additionally, conventional systems and techniques fail to produce images for certain specimen features. For example, the bottom of a contact or trench region is typically undifferentiated from the adjacent sidewalls of the contact or trench. Typically, both the bottom and sidewalls will appear dark because a significant number of the secondary electrons within the contact or trench hit the sidewalls and fail to escape from the contact or trench and reach the detector 108. As a result of this failure, the bottom and sidewall's individual dimensions and shapes are obscured within the resulting image.
Thus, microscopy apparatus and techniques for improving image quality are needed. More specifically, mechanisms for controlling charge distribution on the surface of the specimen are needed.